The following abbreviations are used in the description below:3GPPthird generation partnership project\BSRbuffer status reportCQIchannel quality informationDLdownlinke-NodeBNode B of an E-UTRAN systemeNBrrelay-enhanced eNBE-UTRANevolved UTRANLTElong term evolution of 3GPPNode Bbase station or similar network access node, including e-NodeBPRBsphysical resource blocksRANradio access networkRBradio bearerRBGradio bearer groupRBRradio band resourceRNrelay nodeRSrelay stationQoSQuality of ServiceUEuser equipment (e.g., mobile equipment/station)ULuplinkUMTSuniversal mobile telecommunications systemUTRANUMTS terrestrial radio access network
3GPP is standardizing the long-term evolution (LTE) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. Future LTE standard releases (here termed release 9 for brevity) may use relay nodes (RNs), alternatively termed relay stations (RSs), in order to enhance coverage areas in the cell of an e-NodeB. FIG. 1 gives an overview of how such RNs can be used to advantage: to extend wireless coverage to the interior of a building, to extend coverage beyond the cell edge (cell edge as defined by the radio range from the e-NodeB), to direct radio signals more particularly to a valley between buildings or to a radio-frequency ‘shadow’ behind a building, to extend the cell to other non-contiguous areas such as via multi-hops/multi-relays, and to provide robust radio signals in any other ‘coverage holes’ that may be at different areas of the cell. The RNs can be fixed or mobile, such as mounted to a high-speed train. In some systems the relay stations may be opportunistically available UEs/mobile terminals that are not owned by the network itself. For clarity, a network access node that employs RNs is termed a relay enhanced access node, or in the context of LTE it is termed an e-NBr for brevity.
Apart from this main goal of coverage extension, introducing relay concepts can also be used to aid in the provisioning of high-bit-rate coverage in high shadowing environment, to reduce the average radio-transmission power at the UE which extends the UE's battery life, to enhance cell capacity and effective throughput (e.g., by increasing cell-edge capacity and balancing cell load), and to enhance overall performance and deployment cost of the radio access network RAN.
After being carefully considered in pre-standardization activities like the Wireless World Initiative WWI/Wireless World Initiative New Radio WINNER system concept, relay systems are achieving the level of maturity that is needed in ongoing LTE standardization activities. The WINNER concept seeks to develop a system that is QoS aware and uses intelligent scheduling to meet user demands and physical constraints, in a model that is scalable for deployment to any of various spectrum types and bandwidths including spectrum sharing. This scalable deployment gives rise to relay nodes being an important part of the WINNER concept. As an example of later stages of relay node development, the IEEE 802.16j standardization adds relays to the IEEE 802.16e standard. This recent development has increased the pressure to consider relays also in LTE standardization. Certain wireless network operators have been actively pushing for relay standardization since it is expected that relay systems will be economically viable due to reduced backhaul and site acquisition costs. In order to keep LTE competitive it is more than probable that relay extensions to LTE are to be studied within 3GPP in the release 9. Various topics will be studied and relays appear likely to play an important role in LTE Release 9, and RSs are likely to be included in the LTE Release 9 agenda.
There are many specific types of relay systems proposed, from the simple (e.g., amplify/forward implementations, applied in single frequency networks such as digital video broadcast for handhelds DVB-H, for example) to the more complex such as using network coding to improve the overall performance. A common relay type that is proposed for cellular relaying is a detect/forward type of relay, where an input signal is detected and retransmitted using the same procedure as in the original transmission. The following discussion assumes the detect/forward type implementation for a relay network.
To ensure economic viability in adopting RNs into the LTE network, backward compatibility between Release 8 (standardization ongoing) and Release 9 will be needed. A reasonable assumption is that full backward compatibility is required from the UE perspective, i.e. Release 8 and Release 9 terminals should work equally well in Release 8 and in Release 9 networks. At the network side software and even hardware updates between standard releases may be possible but preferably they should be as small as possible. Hence, from the UE viewpoint the serving network node should function in exactly the same way as the e-NodeBs of Release 8. Due to this requirement, the reduction of functionalities of the e-NodeB when defining and implementing relay nodes will be difficult, and the relay nodes will need to support all of the main e-NodeB functions also. Due to this fact it can be assumed that relay nodes are capable of flexible resource sharing with the e-NodeB that controls them.
The introduction of RNs can have an impact on the overall architecture of the network as well as the UL scheduling and the exchange of buffer status report (BSR) noted above. The scheduling of UEs under control of a RN can be done by the RN with the help of the controlling e-NodeB (e.g., where the RN is given some authority to schedule radio resources given by the e-NodeB), or solely by the controlling eNBr (where the RN acts as a communication conduit and all scheduling decisions are by the e-NodeB).
Certain problems arise in the former case. The RN may be doing the scheduling, but it may still be advantageous that the controlling eNodeB have some information as to buffer status as well as the quality of the UE's UL access links so as to schedule resources (those it reserves to itself and those it allocates to the RN) to efficiently meet the demands on both the UE's access links to the RN and also the RN's relay link to the e-NodeB.
In Release 8 of LTE, uplink UL BSR, referring to the amount of buffered data in the logical channel queues in UE (which may be in the Medium Access Control or higher logical protocol layer), are needed in order to provide support for quality of service (QoS)—aware packet scheduling. Radio bearers (RBs) that have similar QoS requirements are grouped under a radio bearer group (RBG), and currently proposals to LTE are for a total of four RBGs. The UE sends a BSR of the RBGs to the e-NodeB. The e-NodeB then considers the BSRs from the UEs that it is serving, the channel quality that each UE is experiencing in the UL (from UL sounding measurements), and schedules the different UEs accordingly.
Each RB in the UE is given a priority, a prioritized bit rate (PBR) and a corresponding maximum bit rate (MBR). The UE tries to schedule all RBs (within the limits of the granted resources from the eNB) in decreasing priority up to their PBR. Once this is done and if the UE has sufficient resources to satisfy all the PBRs, the remaining resources assigned by the grant (i.e. MBR-PBR) are scheduled for each RB, also in decreasing priority. Note that while the scheduling at the e-NodeB is done on a per RBG basis, the final scheduling at the UE is on a per RB basis.
A straightforward implementation of BSR from RN to e-NodeB would simply convey the status of the actual RN buffer. This is not seen as optimal for several reasons. Having the RN simply forward to the controlling e-NodeB each individual BSR report that the RN receives from the relayed UEs will result in fairly high control signaling overhead, which by these teachings will be seen to be unnecessary for the case where the RN is the one responsible for scheduling its UEs. For similar reasons, having the RN simply relay to the e-NodeB each of the CQI values (which are measured by the RN itself) for each of the UE-RN links will also be shown to use an unnecessarily high amount of control signaling overhead.
The introduction of RNs is a new concept in LTE. Thus, UL scheduling and associated BSR reporting has not been considered previously. The case where RNs control the scheduling of its own users is similar to the non-transparent mode defined in WIMAX. However, the issues regarding UL scheduling and BSR reporting are not yet resolved since those are different in LTE.
What is needed in the art is an approach to optimize scheduling of UEs under control of a RN for various actual channel conditions that exist between the RN and the UEs under its control, preferably in a manner that is consistent with LTE.